Angiogenesis contributes to various pathological conditions. Due to the resistance against existing antiangiogenic therapy, an urgent need exists to understand the molecular basis of vessel growth and to identify new targets for antiangiogenic therapy. Here we show that cyclin-dependent kinase 5 (Cdk5), an important modulator of neuronal processes, regulates endothelial cell migration and angiogenesis, suggesting Cdk5 as a novel target for antiangiogenic therapy. Inhibition or knockdown of Cdk5 reduces endothelial cell motility and blocks angiogenesis in vitro and in vivo. We elucidate a specific signaling of Cdk5 in the endothelium; in contrast to neuronal cells, the motile defects upon inhibition of Cdk5 are not caused by an impaired function of focal adhesions or microtubules but by the reduced formation of lamellipodia. Inhibition or down-regulation of Cdk5 decreases the activity of the small GTPase Rac1 and results in a disorganized actin cytoskeleton. Constitutive active Rac1 compensates for the inhibiting effects of Cdk5 knockdown on migration, suggesting that Cdk5 exerts its effects in endothelial cell migration via Rac1. Our work elucidates Cdk5 as a pivotal new regulator of endothelial cell migration and angiogenesis. It suggests Cdk5 as a novel, pharmacologically accessible target for antiangiogenic therapy and provides the basis for a new therapeutic application of Cdk5 inhibitors as antiangiogenic agents.